1. Field of the Invention
This invention relates to a process for selectively separating gaseous components from a gaseous mixture by using an adsorbent.
2. Description of the Prior Art
As processes for the selective separation of gaseous components, especially gaseous impurity from a gaseous mixture, it has heretofore been the common practice to have the gaseous impurity adsorbed on an adsorbent in an adsorbent layer and, when the adsorbent has reached the limit of its adsorbing ability, to depressurize the adsorbent layer, followed by purging of the adsorbent layer with a gas which does not contain the gaseous impurity so much, so that the gaseous impurity is desorbed from the adsorbent to regenerate the adsorbent.
Among these processes, Japanese Patent Publication No. 47051/1987 (hereinafter abbreviated '051) discloses the following process as a gas separation process capable of minimizing the loss of the treated, i.e., purified gas.
According to this conventional process, the gaseous impurity is selectively and adiabatically adsorbed on an adsorbent. The adsorbent is depressurized and is then purged at a low pressure using a gas which may range from a gas contaminated only a little to a substantially pure gas, thereby conducting desorption of the gaseous impurity and regeneration of the adsorbent. The adsorbent is pressurized again. The conventional process features alternate and cyclic use of many columns with adsorbents so that gaseous impurity is separated from a gaseous mixture to provide a purified gas. Each cycle comprises a number of steps starting with a first adsorption column whose adsorbent has fatigued as a result of adsorption of the gaseous impurity. These steps comprise
depressurizing the first adsorption column through an outlet thereof while an inlet of the first adsorption column is closed, so that gas being present in voids of the first adsorption column is allowed to expand;
introducing the thus-expanded gas to another adsorption column, which has been regenerated, through an outlet of said another adsorption column until said another adsorption column and the first adsorption column are equalized in pressure;
depressurizing the first adsorption column further through the outlet thereof to cause the gas, which is present in the voids of the first adsorption column, to expand further and, then, introducing the further-expanded gas into a packed column with an inert, nonporous column packing having a high void fraction from one end thereof;
introducing another further-expanded gas from a second adsorption column, whose adsorbent has fatigued, to the packed column through the opposite end of the packed column, whereby eliminating the first-mentioned further-expanded gas from the packed column;
depressurizing the first adsorption column to a lowest pressure through the inlet thereof, introducing a portion or all of the first-mentioned further-expanded gas, which has been eliminated from the packed column, into the first adsorption column to purge the first adsorption column, and if there is still any remaining portion of the first-mentioned further-expanded gas eliminated from the packed column, introducing the remaining portion of the first-mentioned further-expanded gas into a third adsorption column, which has been regenerated, through an inlet thereof to pressurize the third adsorption column to an intermediate level;
introducing expanded gas from a fourth adsorption column, which has fatigued, into the first adsorption column, which has already been regenerated, through the outlet while closing the inlet of the first adsorption column until the fourth adsorption column and the first adsorption column are equalized in pressure;
introducing a gas of the same quality as the purified gas from a gas stream of the same quality as the purified gas into the first adsorption column through the outlet thereof while closing the inlet of the first adsorption column until the pressure inside the first adsorption column becomes equal to the pressure of the gas stream; and
introducing the gaseous mixture, which contains the gaseous impurity, into the first adsorption column through the inlet thereof and discharging the purified gas through the outlet of the first adsorption column.
The above conventional process has the merit that the loss of the purified gas can be reduced. Due to the use of the packed column, which is packed with the packing of the high void fraction, as a gas holding column, it is, however, accompanied by practical problems to be described next.
Preparation or purchase of the column packing to be placed in the packed column is not easy and, further, takes a rather long time. It is also necessary to pack the column packing on the spot. The column packing results in a substantial increase in both weight and cost of the packed column. Further, the column packing must be taken out and then packed back upon inspection of the inside of the packed column, thereby requiring time and cost.